The invention relates to a growing prosthesis comprising a bone prosthesis, in particular a joint replacement part, a prosthesis stem, a stem-anchoring element, and an element with corresponding drive provided for the lengthening of the growing prosthesis.
A number of growing prostheses are known from the prior art.
Growing prostheses in most cases consist of a lengthenable prosthesis part and of a stem-anchoring element, which is fixed in the bone. In devices of this kind, an electric drive is located in the prosthesis part. Spindle roller systems are mostly used as the electric drive. A drive unit of this kind is disclosed in EP 0 776 432 B1. The device converts a rotary movement into an axial movement using planetary rollers, which are driven by a drive shaft out of a stationary drive housing and held peripherally at uniform spacings, and a pusher body that can be axially displaced by the planetary rollers.
On account of the movement of the drive unit, the telescopic element of the prosthesis is moved out and the prosthesis is lengthened. The stem-anchoring element can have different lengths and cross sections and can be of modular construction, such that it can be adapted to the different medullary cavity dimensions.
EP 1 371 346 B1 discloses, for example, an implantable prosthesis for replacement of the human hip joint or knee joint and of the adjacent portions of the diaphysis, wherein the prosthesis has a joint part, a stem-anchoring part and, arranged between the joint part and the stem-anchoring part, a stem replacement part. The stem replacement part and the artificial joint each have a continuous central bore, said bores being aligned with each other. The stem-anchoring part is formed in the present invention by an intramedullary distraction pin and is fitted into the central bores. After the desired lengthening of the bone by means of callus distraction, i.e. formation of new bone tissue, such intramedullary pins in most cases have to be removed again and replaced by a stablilizer, since the intramedullary pins do not have sufficient stability for permanent implantation of the prosthesis.
Moreover, DE 199 06 423 A1 discloses an active intramedullary pin which is used for distraction of bone parts and which consists of two elements, prostheses or the like that are movable relative to each other and have at least one electrically operated drive element. The electrical energy needed for the drive element is delivered to the intramedullary pin via at least one releasable plug element. Should the intramedullary pin be integrated in the prosthesis, it is of particular advantage in this invention that, after the desired distraction has been completed, only the electric plug connector has to be removed, not the entire intramedullary pin. However, there is still the problem in this invention that, because of the predefined length, a not inconsiderable portion of healthy bone has to be removed for introducing the prosthesis.
The disadvantage of the identified prior art is, on the one hand, the relatively large diameter of growing prostheses that comprise planetary roller drives. Such prosthesis models are suitable only to a limited extent in the medical treatment of hip joints and knee joints that are to be replaced in children and adolescents. The long minimum length of the prosthesis, which is predefined by the relatively long length of the drive, is also a considerable disadvantage since, in children with smaller bones, the long and predefined length of the prosthesis means that more bone than necessary has to be removed in order to create the space for receiving the prosthesis that is to be implanted.
It is therefore the object of the present invention to provide a growing prosthesis which diverges from the principle of callus distraction and which at the same time has a reduced prosthesis length and a reduced prosthesis diameter, so as to permit easier use of such prostheses in children and adolescents or in persons of small stature.